Carbon and graphite ceramics with grain sizes smaller than 20 μm are as a rule produced from two-component systems with a solid (coke) and a binder. To improve the property profile, additives are usually employed, which increase the coking residue of the binder. Two-component materials have the main drawback that after forming during carbonisation, the coke used expands, whereas the reinforcing binder framework shrinks. Shrinkage of both constituents does not occur until the pre-treatment temperature of the coke is exceeded. This fundamental problem explains the formation of microcracks in the structure and the limitation of the strengths attainable in the finished ceramic using this technology.
A new route employed sinterable semicokes such as CARBOSINT®, production of which is described in EP 0 552 422 B1. Semicoke is a carbon material, which is intermediate between a fusible pitch and an unfusible green coke. It can be obtained by incomplete carbonisation at a temperature between the initial melting of the pitch and 500° C. optionally in combination with oxidation at below 250° C. Optimally matched semicokes serve simultaneously as filling coke and binder in ceramics manufacture. Owing to the sticky surface, semicoke powder of this kind can be processed without further additives by die pressing or isostatic pressing. Although sinterable semicokes have long been available, and despite their advantageous property profiles, these single-component materials have so far not been widely adopted commercially, for the reasons explained below.
The advantage of the stickiness of the coke for its compaction means at the same time that such a material cannot be handled easily, because it tends to form lumps and to adhere in the feeding systems to automatic presses or during die filling. Therefore homogeneous die filling is far more difficult and involves considerable expense.
Another disadvantage of this coke is the low bulk density of about 0.4 g/cm3. A component based on fine-grained carbon as a rule has a geometric density of 1.2 to 1.3 g/cm3 after forming, which means that in die pressing it must be compacted by a factor of at least 3. A doubling of density can be controlled by the equipment. Higher compaction factors limit—even in the case of multilevel axial presses—the complexity and design freedom of the components that can be produced.
In the manufacture of components by isostatic pressing, the great stickiness of the coke makes homogeneous filling of the flexible die more difficult. Density gradients in the component resulting from inhomogeneities during filling cannot, however, be corrected in subsequent processing.
These disadvantages still hamper many possible applications, which require forming that is close to the final shape. Mechanical post-processing is generally too expensive. This impedes full utilisation of the potential of sinterable semicoke with respect to the product properties of technical carbon products.
To overcome these disadvantages and thus produce sinterable semicoke powder for the production of carbon and graphite ceramics close to the final shape, U.S. Pat. No. 4,985,184 proposes suspending the carbon powder with binder in a proportion from 0.01 to 5 wt. % and a non-ionic wetting agent in an amount from 0.01 to 3 wt. %, in each case relative to the mass of the solid matter, and then granulating by spray-drying. Methylcellulose or hydroxycellulose is proposed as binder, and alkylphenol ethylene oxides as wetting agents. A disadvantage of this method is the low solids content of the suspension of only 33%, necessitating high water evaporation rates during spray granulation, which is therefore associated with high process costs. Another disturbing factor in further processing is the large amount of additives of up to 8 wt. %.